Increasingly the prescription surfaces of ophthalmic lenses have a so-called “freeform” geometry, such as that used in progressive addition lenses (PALs). Freeform optical surfaces are defined as any non-rotationally symmetric surface or a symmetric surface that is rotated about any axis that is not its axis of symmetry. Current state of the art in freeform lens curve generating technology offers only a few different options. These options are three dimensional lens milling, three dimensional lens grinding and three dimensional lens turning.
Three dimensional lens milling can be described as a simple rotating tool with a single or multiple attached cutter blades spinning at a relatively high rotational speed. The tool is moved relative to the desired lens surface in using at least three axes of motion. Each time a cutter blade cuts into the lens surface, a small “bite” is taken out of the surface, leaving behind a slightly scalloped surface, but of the desired general curve geometry. Such process is described in, e.g., document EP-A-0 758 571 by the same applicant. Although a very good cutting rate and consequently short machining times that meet industrial requirements can be obtained with this known method, it would be desirable, in certain applications, to obtain an even better surface quality, particularly in the case of complex optical surfaces, such as freeform surfaces.
To this end document EP-A-1 291 106 by the same applicant proposes a method for the surface machining of in particular plastic spectacle lenses, which method starts with a three dimensional lens milling step, and finishes with a turning step to remove the “scallops”, and improve the surface finish. The turning step however adds to the machining time.
An infinitely high spindle speed, or an infinitely high number of cutter blades mounted to the tool and perfectly aligned relative to the axis of rotation would provide infinitely small “bites” out of the surface, and therefore provide a surface with improved quality, i.e. one without the scalloped appearance. A grinding wheel can be thought of as a tool having an infinite number of cutters, however grinding does not work very well with plastic materials.
In three dimensional lens grinding, a grinding wheel of similar general geometry to that of the milling tool described above is positioned according to the same three axis tool motion path to achieve the same lens shape as that achieved with the milling tool. Grinding however typically works well for hard brittle materials like mineral glass, but not so well for soft ductile materials like most plastics. The soft materials tend to adhere to the grinding wheel which loads the grinding surface and prevents further cutting.
Three dimensional lens turning, also called “Fast Tool Single Point Diamond Turning” (SPDT), is currently the technology of choice to obtain high quality surface finish at relatively high speeds. As becomes apparent from, e.g., document WO-A-02/06005 by the same inventor, this technology uses a fast moving, short travel turning tool, controlled at high frequencies, and synchronized in motion to the work piece turning spindle, and the cross axis position, to obtain the desired freeform shape. One limitation to this approach is the surface speed of zero at the center of the lens, creating undesirable “center features”, as described in European patent application 05 009 894.6 by the same applicant. Precise tool calibration is required to minimize such undesirable “center features”, however the zero surface speed and other geometry issues at center make it difficult to completely eliminate all undesirable “center features”.
Two other well known generating technologies generally considered to be not capable of generating freeform shapes are cup wheel grinding and “Single Point Diamond Fly Cutting” (SPDFC):
Cup wheel grinding is a method used with hard brittle materials to achieve excellent surfaces on spheres, rotationally symmetrical aspheres, and toric surfaces. The cup wheel tool is maintained in contact with the lens surface for its entire rotation, therefore providing better surfaces. Such process is described in, e.g., documents U.S. Pat. No. 4,866,884 and U.S. Pat. No. 5,181,345. Again, cup wheel grinding typically works well for hard brittle materials like mineral glass, but not so well for soft ductile materials like most plastics which adhere to and load up the cup wheel tool.
Very similar in geometry and therefore curvature limitations to cup wheel grinding described above is SPDFC. On organic (plastic) materials SPDFC is capable of providing one of the best surface qualities of all the technologies listed to date. On standard toric and spherical surfaces the relative surface speed of the tool—the fly cutting tool is a single-point cutting tool similar to a lathe tool mounted in a special rotating holder—is maintained to be very constant, and relatively high. An elliptical toroidal shape is obtained when cutting toric curves. This toroid is different from a true toric shape and is therefore said to have “elliptical error”. Examples of fly cutting tools can be found in document U.S. Pat. No. 5,919,013 by the same inventor and document U.S. Pat. No. 5,704,735.
What is needed is an improvement in the machining quality while maintaining acceptable machining times for cutting ophthalmic lenses with freeform optical surfaces.
What is also needed is an apparatus and an efficient method, by means of which optical surfaces having in particular a freeform geometry can be generated with high surface quality and at appropriate cutting rates.